Some electronic parts and electronic materials which are incorporated into electronic equipment do not have heat resistance and deteriorate in function or break when exposed to high temperatures. Such electronic parts and electronic materials must be soldered at a low temperature. For this purpose, a solder having a low melting point, i.e., a low-temperature solder must be used. There is no precise definition of a low-temperature solder, but in general it indicates one having a solidus temperature and a liquidus temperature (melting point) which are lower than 183° C., which is the eutectic temperature of Pb-63Sn alloy.
Conventional low-temperature solders include Sn-52Bi-32Pb (melting point of 95° C.), Sn-54Bi-20Cd (melting point of 103° C.), Sn-40Pb-40Bi (melting point of 113° C.), Sn-52In (melting point of 117° C.), Sn-58Bi (melting point of 139° C.), Sn-32Pb-18Cd (melting point of 143° C.), Sn-32Cd (melting point of 175° C.), and the like.
The solidus temperature and liquidus temperature of a solder alloy can be adjusted by adding a suitable amount of Pb or Cd, thereby making it possible to obtain low-temperature solders having various melting points. However, as low-temperature solders containing Pb or Cd cause environmental problems in that they produce adverse effects on humans, their use has come to be regulated. Accordingly, there is a demand for low-temperature solders which do not contain Pb or Cd.
Electronic parts equipped with semiconductor elements such as Peltier elements which have started to be used in recent electronic equipment has a heat resisting temperature of at most 130° C. Therefore, they have low heat resistance compared to typical electronic parts. When electronic parts having such low heat resistance (referred to below as low heat resistance parts) are soldered, the soldering temperature must of course be lower than 130° C.
In general, an appropriate soldering temperature is considered to be 20-40° C. higher than the liquidus temperature of the solder being used. Accordingly, low-temperature solder used for soldering low heat resistance parts must have a liquidus temperature of around 100° C., i.e., in the range of 90-110° C.
The temperature of use of typical electronic equipment is from −40° C. in Arctic regions to +85° C. in direct sunlight in the tropics. Therefore, a low-temperature solder used for soldering of low heat resistance parts must have sufficient resistance to thermal fatigue from −40° C. to +85° C., and its solidus temperature must be at least 85° C. so that the solder does not melt at the highest temperatures to which electronic equipment is exposed.
Because solder bonds two metals (base metals) to each other through a soldered joint, various bonding characteristics are required of solder. First, for example, solder must have solderability such that it can easily wet the base metals. Even if its solderability is not extremely good as can be obtained with conventional Pb-63Sn solder, it must be solderability of a level that does not impede the production of electronic equipment.
Solder are also required to form soldered joints having excellent mechanical properties (tensile strength, bending properties, elongation, etc.). Namely, when a tensile force is applied to a base metal which was soldered, if the soldered joint easily peels off, electronic equipment becomes completely unable to function. Similarly, when a bending force is applied to a base metal of a soldered joint, the solder must have sufficient ductility to prevent it from easily breaking by surrendering to the bending force.
Solder also requires corrosion resistance. When soldered electronic equipment is only used indoors, corrosion is not a problem because there is not much of a difference between cold and hot and the environment is good. However, electronic equipment used in equipment for data communication stations, automobiles, military equipment, equipment for outer space, equipment for outdoor recreation, and the like is often disposed outdoors, where corrosion of solder becomes a problem. Outdoors, there is a large temperature difference between day and night. If electronic equipment is exposed to a high temperature during the day and a low temperature at night, humid air in the periphery of soldered joints during the day causes moisture to condense on soldered joints at the low temperature at night. Since solder and base metals are made of different metals, they naturally have different ionization tendencies, and the condensed moisture forms an electrolytic solution, resulting in the formation of a local cell between the base metals and the solder. As a result, the base metals or the solder ends up electrically corroding. Corrosion of a soldered joint initially affects the electrical conductivity thereof. As the corrosion proceeds, conduction is completely lost between the base metals, and the electronic equipment can no longer perform its function.
Summarizing the above, the conditions now demanded for a low-temperature solder for soldering of low heat resistance parts are as follows.
(1) The solder does not contain Pb or Cd.
(2) Its liquidus temperature is at most 110° C., and its solidus temperature is at least 85° C.
(3) The solder has resistance to thermal fatigue from −40° C. to +85° C.
(4) It has appropriate solderability.
(5) The solder has appropriate mechanical properties such as mechanical strength, bending properties, ductility, and elongation.
(6) It is not susceptible to corrosion.
A Sn-58Bi based low-temperature solder which has been used from in the past for soldering of electronic equipment has a liquidus temperature of 139° C. Therefore, it cannot be used for soldering of low heat resistance parts like those described above (which have a heat resistance temperature of 130° C.). A Sn-521n based low-temperature solder has a solidus temperature and liquidus temperature of 117° C., but this solder does not have sufficient solderability or resistance to thermal fatigue, so it had problems with respect to reliability.
Patent Document 1 proposes a Bi—In based low-temperature solder. One example of the solder disclosed therein has a composition in the range of 50-83 atomic % In (35.4-72.8 mass %) and a balance of Bi. The low-temperature solder disclosed in Patent Document 1 is for joining wiring patterns of optical heads and optical modules. This Bi—In based low-temperature solder is formed into a powder and mixed with a flux for use as a solder paste.
Patent Document 1: JP 2001-198692 A1